Wave guide attenuator



July 1952 w. H. HEWITT, R 2,602,857

WAVE GUIDE ATTENUATOR Filed Aug. 24, 1944 lNl/ENTOR By WH. HEWITKJR.

A TTORNEY Patented July 8, 1 952 UNITED STATE WAVEGUIDE ATTENUATOlt William H. Hewitt, Jr., Flushing, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. ,Y., a corporation of New York Application August 24, 1944, Serial No.'551,040

Claims. (01. 178-44) This invention relates to wave transmission maximum attentuation obtainable in a variable wave guide attenuator.

Another object is to improve the frequency characteristic of such an attenuator, that is,'to decrease the change in attenuation resulting from a change in the frequency.

A further object is'to reduce the reflection at the junction between an attenuator and a wave guide.

Further objects of the invention are to reduce the size and weight of a wave guide attenuator providing a given maximumattenuation.

The variable wave guide attenuator of the present invention comprises a section of metallic pipe wave guide in which are longitudinally and symmetrically positioned a pair of resistive vanes which may be simultaneously moved from the sides of the guide towards the center or back again, as required. The vanes are arranged with their major faces parallel to the direction of the electric field of the waves to be attenuated. If the guide is of rectangular cross-section with unequal cross-sectional dimensions the vanes are, therefore, parallel to the shorter side. By means of two racks and a pinion gear the vanes may be moved laterally in opposite directions but through substantially equal distances. In order to reduce reflection and improve the frequency characteristic the vanes may have a number of holes extending through them and tapered ends.

As compared with a single-vane attenuator,

the two-vane attenuator has an increase in maximum attenuation of more than 40 per cent with no increase in size and without a corresponding increase in weight, a decrease in the change in attenuation with frequency which, over a typical range, may be of the order of 2.9 per cent compared to 5 per cent, and an improved impedance match with the associated guide, reducing reflection at the junctions.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings, in which like reference characters refer to similar or corresponding parts and in which:

Fig. 1 is a top view of a variable wave guide attenuator in accordance with the invention;

Fig. 2 is an end view, partly in section, taken along the line 2-2 of Fig. 1;

Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2; and,

Fig. 4 is a side elevation. 1

As shown, the attenuator :comprisesa section of metallic wave guide 4 of-rectangularucrosssection: inwhich are longitudinally, and symmetrically-positioned a pair of: resistive-vanes 5. The electromagnetic waves to be attenuated enter atone end, as indicated bythe arrow 6 in Fig. 1, and leave at the-other end,--as in,dicated bythe arrow l; The-,majonfaces of the vanes 5 are parallel to the direction :of the electric field of the waves to be attenuated. In ;a rectangular guide having unequal cross-sectional dimensions, such as ll, this .flelcl-yvill beparallel to the sides 8 having the shorter-dimension, as indicated by the arrow in Fig. 2. The vanes 5 are therefore arranged parallel to the sides of; t he guide 4.;

Unitary means are provided for simultaneously moving the vanes 5 lat'erally -inopposite direc-.- tions but through substantially equal distances from the sides 8 of the guide 4 to a central position and back again in order to get the desired attenuation setting. For example, in the embodiment shown, each of the vanes 5 is mounted on the end of a round hard rubber rod 9 by means of a hard rubber screw In which passes through a central hole in the vane 5 and screws into a tapped hole in the end of the rod 9. Each rod 9 passes through a hole in the side of the guide 4 and through a collar l I in which it is free to slide. To the outer end of each rod 9 is attached a rack holder [2 which is bent so that it rests on the upper side of the wave guide 4 against a guide bar It. To each rack holder 12 is fastened a rack l4 which includes a spring l5 between the rack l4 and the bar l3. The rack holders 12 are further held in place by the plate I6 and the springs IT. The main part of the spring I! is positioned between the plate I6 and the rack holder 12 as indicated by the broken lines in Fig. 3 and the ends are bent back to clip over the sides of the plate It. Intermeshing both of the racks I4 is a pinion gear l8 which is attached to the lower end of a shaft l9. The shaft l9 passes through a bushing 20 which is soldered or otherwise attached to the plate It. At the other end of the shaft I9 is attached a knob 22 and a pointer 23, of transparent material with a hair line 24 on the under side. Beneath the pointer 23 is a calibrated circular dial 25 which is attached to the bushing 20 and is therefore fixed in position. The dial 25 may be calibrated directly in decibels of attenuation. A typical calibration, from zero to 66 decibels, is shown.

The vanes 5 are made of some suitable insulating material such, for example, as phenol fibre,

and the major face nearer the wall 8 of the guide 4 is coated with one or more layers of resistive material 21. The resistive material may, for example, be a mixture of finely divided graphite and a suitable binder, sprayed on in one or more coats. To reduce reflection and improve the frequency characteristic, each end of the vane 2 is tapered for a distance D approximately equal to a half wavelength. A longer taper will reduce the maximum attenuation while a shorter one will degrade the frequency characteristic. holes 26 through the vane 5 also contribute to improving the frequency characteristic by decreasing the change in attenuation resulting from a change in the frequency of the waves. There are preferably two or more of these in each half of the vane symmetrically arranged about the transverse center line. The maximum attenuation obtained is dependent upon the length of the vanes 5 and the thickness of the resistive material 21.

The attenuation introduced by the attenuator depends upon the positions of the vanes 5, which are controlled by the knob 22. When the knob 22 is turned in a clockwise direction until the hairline 24 is over zero on the dial 25, the vanes 5 will lie against the side walls of the guide 4 and there will be practically no attenuation introduced. Now, as the knob 22 is turned in a counterclockwise direction the vanes 5 move simultaneously toward each other through equal distances, continuously increasing the attenuation until a maximum is reached as the vanes approach each other near the center of the uide 4.

What is claimed is:

1. An attenuator comprising a section of wave guide and a resistive vane longitudinally posi- The tioned therein, said vane having four holes therein approximately centered on the longitudinal center line and symmetrically arranged about the transverse center line.

2. An attenuator in accordance with claim 1 in which said holes are circular and each has a diameter equal to at least one-quarter of the width of said vane.

3. An attenuator in "accordance with claim 2 in which the spacing between the centers of the holes on one side of the transverse center line is approximately the same as the spacing between the transverse center line and the center of the hole nearer thereto.

4. An attenuator in accordance with claim 3 in which each of said spacings is approximately equal to the width of said vane.

5. A variable attenuator comprising a section of wave guide, a pair of resistive vanes longitudinally positioned within said guide substantially opposite each other with their major faces substantially parallel to the electric field of the waves to be attenuated, and means for moving said vanes laterally in opposite directions, each of said vanes having four holes therein approximately centered on the longitudinal center line and symmetrically arranged about the transverse center line.

WILLIAM H. HEWITT. Jn.

REFERENCES CITED UNITED STATES PATENTS Name Date Wolfi July 16, 1940 Number 

